There are many applications that utilize accumulator batteries, also known as rechargeable batteries, to store and supply electrical power. These systems employ Direct Current electricity, internally, but may convert external Alternating Current power source to charge the batteries, and may generate Alternating Current at the output to drive load types such as electrical motors, or to supply the output energy to a Alternating Current system. Two of the most typical applications where large capacity accumulator batteries are used are telecommunications sites (including landline Central Office locations, cellular Base Station sites, satellite ground stations etc.), and electric or hybrid vehicles. Battery technology also depends on the application, for example in telecommunications the weight of batteries does not substantially matter, therefore the less expensive Lead-Acid batteries are utilized, while in electric vehicles where weight is of essence, the more advances Lithium-Ion battery technology is used.
This invention is equally applicable to all batteries that store electrical energy in the form of chemical potential energy, and is equally applicable to all applications, which combine a substantial number of large-capacity rechargeable batteries in arrays. If the batteries are small or if there are just a few cells connected in series, per-cell monitoring may not be economically justified, but in cases where large-capacity cells are connected in large arrays to generate a high voltage, these cells carry a significant cost and value, and will justify the monitoring of individual cells.
Why do Batteries Need Monitoring?
Irrespective of application, accumulator batteries have a finite life and in most cases require maintenance. Maintenance may include adding distilled water into telecommunications lead-acid batteries, execution of specific charging profile thereby controlling the flow of current over time in a specific way, or simply the replacement of specific cells in an array which are the weakest and limit the performance of the entire array. For the purpose of maintenance the measure of interest is the battery “health”, and that metric can be deduced from monitoring the Voltage/Current (i.e. VP) profile of a cell and its temperature. For example, with use the amount of electrical energy that can be stored in a cell is reduced, which in turn affects the rate of cell voltage increase during charging. Also, certain types of battery wear increase the resistance of its electrolyte and will increase their temperature for a given current. Therefore for maintenance purposes, one needs to accurately measure the voltage across every cell in the array and its local temperature. These values then can be processed into “health” by combining them with the charge or discharge current flowing through the array, or a “relative health” can be calculated simply by comparing the voltage and temperature values of all the cells in a given array.
The other type of measurement needed in some application is the “state of charge” (SOC), which is the measure of the amount of potential energy stored in a given cell. This information is then used to initiate a charging procedure when the cells are depleted, or to terminate charging when the cells are sufficiently charged. The SOC measurement becomes more valuable when measured for each cell of the array individually rather than for an entire array. In order to maximize the life of batteries, and thus to maximize their economic value, the charge and discharge processes must be guided by the weakest cell in an array, so as to avoid over- and under-charging of the weakest cell and thus maximize its life.
The need for monitoring each cell individually and the need for balancing the performance of cells in an array is widely documented in trade publications, such as in Cell Balancing Maximizes The Capacity Of Multi-Cell Li-Ion Battery Packs by Carlos Martinez, Intersil, Inc.—http://www.analogzone.com/pwrt0207.pdf